The centrifuges used to recover wool wax from scouring liquors must perform a number of functions.

• Remove the wax from the scouring liquor as a cream (oil in water emulsion)
• Remove water-soluble contaminants and fine dirt from the cream by washing with hot water
• Concentrate the solids content of the cream
• Produce anhydrous wool wax by inverting the emulsion and concentrating the solids content.

Wool wax recovery loop

In a modern plant two or even three primary centrifuges should be used with a high recirculation rate to ensure the best wax recovery. These centrifuges separate the scouring liquor into three phases, a cream phase containing the recovered wool wax, a sludge phase containing dirt and a centrate that can be recycled to the scouring line. They are protected usually by hydrocyclones to prevent abrasive dirt particles from damaging the centrifuges. Disc stack centrifuges fitted with nozzles are preferred to those with opening bowls to control the discharge of the dirty phase. These nozzles have a hole diameter of 0.8 to 1.2mm and without protection can easily block or become eroded.

In a modern scouring plant a three-stage system is used. The cream phase from the primary centrifuges should contain 15-20% wax. This weak cream is stored in a heated tank before being washed and concentrated to about 70% through concentrating centrifuges. The concentrate is again stored in a heated tank before being fed to a purifier centrifuge in which the emulsion is inverted to a water-in oil emulsion and concentrated to an anhydrous product containing low levels of water, dirt and volatile material.

Some mills use a two - stage recovery system in which the primary cream has to contain about 45% solids in order to obtain an anhydrous product. The second centrifuge step has to wash the cream, concentrate it and invert the emulsion - all in one step. This makes it very difficult to optimise. Three-stage systems are more preferable and the overall yields of recovered wool wax are higher.